1. Field of the Invention
The present invention relates to an optical circuit device and a method for fabricating the optical circuit device, more specifically to an optical circuit device including a Faraday rotation element, and a method for fabricating the same.
2. Description of the Related Art
An optical isolator propagates light in one direction (forward direction) substantially without attenuation and does not propagate the light in the opposite direction (backward direction) and is a non-reciprocity optical device which propagates light in one direction alone.
A beam exiting a semiconductor laser is reflected outside the semiconductor laser to enter again the semiconductor laser, which makes the operation of the semiconductor laser unstable, resulting in noise increase. For removing such reflected beam, the optical isolator is disposed on the output side of a semiconductor laser.
A conventional optical isolator will be explained with reference to FIG. 10. FIG. 10 is a conceptual view of the conventional optical isolator.
As shown in FIG. 10, the optical isolator comprises a Faraday rotation element 122, polarizers 119a, 119b disposed, sandwiching the Faraday rotation element 122, and permanent magnets 120a, 120b. 
In the optical communication, light of long wavelength regions, as of a 1.3 μm-band and a 1.55 μm-band is used. As the Faraday rotation element 122 for such long-wavelength region, bulk yttrium iron garnet (YIG) is generally used.
Of forward light incident on the polarizer 119a only a component on the polarization plane of the polarizer 119a passes through the polarizer 119a to be introduced into the Faraday rotation element 122. Forward light introduced into the Faraday rotation element 122 exits through the polarizer 119b because the polarization plane is rotated by 45 degrees due to the Faraday effect.
On the other hand, of backward light, which is reflected light, a component on the polarization plane of the polarizer 119a passes through the polarizer 119a to be introduced into the Faraday rotation element 122. The backward light introduced into the Faraday rotation element 122 does not exit the polarizer because the polarization plane is rotated by 45 degrees in a direction opposite to a direction for the forward light, and the polarization plane of the polarizer is offset by 90 degrees.
Thus, the optical isolator can transmit light in only one direction.
As an optical element using the Faraday rotation element, an optical circulator is proposed.
In the optical circulator incident light and exit light circulate, and the optical circulator is a non-reciprocity optical device having the function of isolating the incident light and the exit light from each other.
A conventional optical circulator will be explained with reference to FIGS. 11A and 11B. FIGS. 11A and 11B are conceptual views of the conventional optical circulator. FIG. 11A is a conceptual view of a structure of the conventional optical circulator. FIG. 11B is a conceptual view of an operation of the optical circulator shown in FIG. 11A.
As shown in FIG. 11A, the optical circulator comprises a Faraday rotation element 122, a half-wave plate 123, polarization beam slitters 125a, 125b, and mirrors 127a, 127b. 
In such optical circulator, as shown in FIG. 11B, light incident on a port 1 exits only at a port 2, light incident on the port 2 exits only at a port 3, light incident on the port 3 exits only at a port 4, and light incident on the port 4 exits only at the port 1.
On the other hand, recently optical circuit devices comprising optical elements, such as a semiconductor laser, semiconductor receiving optics, an optical modulator, a semiconductor light amplifier, an optical multiplexer, an optical branching filter, etc., formed on one and the same substrate is proposed. All of such optical elements can be formed of compound semiconductors, and can be integrated on one and the same compound semiconductor substrate. In integrating semiconductor lasers of a 1.3 μm-band and a 1.55 μm-band used in optical communication, III–V group compound semiconductor substrates, such as InP substrates, InGaAs substrates, GaAs substrates, etc., are used.
However, yttrium iron garnet the above-described Faraday rotation element 122 is formed of is a material which is very difficult to be used on III–V group compound semiconductor substrates, such as InP substrates, etc. In a case where the Faraday rotation element 122 is formed of yttrium iron garnet, the Faraday rotation element 122 and a semiconductor laser, etc. cannot be integrated on one and the same compound semiconductor substrate.
Recently, as materials of the Faraday rotation element, the use of II–VI group magnetic semiconductor and III–V group magnetic semiconductor containing MnAs are proposed.
However, the Faraday rotation element of the proposed II–VI group magnetic semiconductor is usable only for the light of a short wavelength region and is not usable for the light of a long-wavelength region as of a 1.3 μm-band and a 1.55 μm-band. In a case where II–VI group magnetic semiconductor is grown on a III–V group compound semiconductor substrate, the II–VI group magnetic semiconductor cannot grown to have good crystallinity, with a result of large light loss. Accordingly, when a material of the Faraday rotation element is II–VI group magnetic semiconductor, it is difficult to provide an optical circuit device comprising the Faraday rotation element and a semiconductor laser integrated on one and the same III–V group compound semiconductor substrate.
The III–V group magnetic semiconductor containing MsAs has large photoabsorption, has low Curie temperature, and does not have good crystallinity. When a material of the Faraday rotation element is MnAs content-III–V group magnetic semiconductor, it is difficult to provide an optical circuit device comprising the Faraday rotation element and a semiconductor laser integrated on one and the same III–V group compound semiconductor substrate.